1. Field of the Invention
Embodiments of the present invention relate to a semiconductor device and a light-emitting device, and more specifically, to a semiconductor device including a transistor and a light-emitting device such as an organic electroluminescence display device using the semiconductor device as a drive circuit.
2. Description of the Related Art
A semiconductor device including a transistor is used in a pixel circuit of a liquid crystal display device and an organic EL display device. The pixel circuit is provided for each pixel of a display device. Each pixel circuit drives a pixel of a liquid crystal element or an organic EL element.
FIG. 2 shows a pixel circuit of a typical organic EL display device. The pixel circuit includes an organic electroluminescence element (hereinafter referred to as “organic EL element”) EL, a drive transistor Tr1 that supplies power to the organic EL element, a selection transistor Tr2 that turns on during a selection period and obtains data, a capacitor Cap that holds data, a data line L1, a selection line L2, and a power source line L3, and the like. The pixel circuit is connected to one electrode of the organic EL element, which is the anode. The cathode, which is the other electrode of the organic EL element, is fixed to a constant potential, which is the ground potential.
When the selection transistor Tr2 is turned on by the selection line L2, a signal transmitted from the data line L1 is held by the capacitor Cap. The voltage held by the capacitor Cap determines current flowing through the drive transistor Tr1. When the current flows through the organic EL element, the organic EL element emits light. The magnitude of the current determines the intensity of emitted light.
The voltage held by the capacitor Cap is changed by leakage current of the selection transistor Tr2 when the selection transistor Tr2 is turned off, so that the change of the voltage changes the current of the drive transistor Tr1 and changes the light emitting state of a display panel. When the degree of the change of the light emitting state is significant, bright spots and dark spots appear, so that the display quality degrades. Therefore, even when some leakage current occurs, the voltage between both terminals of the capacitor Cap needs to be hardly changed.
Japanese Patent Laid-Open No. 2003-241687 proposes a liquid crystal display device in which, when a holding capacitor Cap of a liquid crystal element is formed of an interlayer insulating film between a gate electrode layer and a drain electrode layer, to obtain a large capacitor Cap, the thickness of the interlayer insulating film in an area where the upper and the lower electrode layers overlap each other is thinned.
In a liquid crystal display device, the pixel circuit generally includes the selection transistor Tr2 and the capacitor Cap. On the other hand, in a display device such as an organic EL display device which includes light-emitting elements, light is emitted from the light-emitting elements by constantly flowing current through the light-emitting elements, so that the drive transistor Tr1 for supplying current and the power source line L3 are required in addition to the configuration of the liquid crystal display device. Therefore, space for forming the capacitor Cap that holds data is extremely limited. In an organic EL display device having a high pixel density, the area of one pixel is small, so that it is difficult to form a capacitor having a sufficient capacity.